1. Field of the Invention
The invention is a process for isomerizing paraffinic hydrocarbons with a strong acid, solid catalyst comprised of a sulfated Group IVB metal and at least one Group VIII metal in the presence of hydrogen and an adamantane or adamantane derivative.
2. Background Information
Several catalytic methods have been used for isomerizing hydrocarbons. Many have required high temperatures, expensive or hard to handle catalysts, expensive corrosion-resistant equipment, or complex recovery procedures for carrying out extensive isomerization. The commercial catalysts used in isomerizing hydrocarbons include aluminum halides, hydrogen fluoride, sulfuric acid and the like. However, the selective isomerization of long chain paraffins is not practiced with these catalysts because it is difficult to control side reactions. Aside from this, it has long been recognized that sulfuric acid and halide-containing catalysts may cause environmental problems, since in many instances they are highly corrosive and not easily disposable. What is ultimately desired is a catalyst that can achieve both high selectivity and a high degree of branching from long chain normal paraffins (those containing 7 or more carbon atoms) under mild conditions thereby avoiding substantial side reactions, such as cracking. The ideal catalyst would be capable of catalyzing the isomerization reaction at low temperatures by providing strong acid sites for catalysis. Sulfate ion modified zirconia has been found to be a very reactive solid superacid, see Hino, M. et al., Chem. Lett., pp. 1671-1672 (1981). U.S. Pat. No. 3,032,599 describes a process for isomerizing C.sub.5 - normal paraffins using sulfate ions to enhance the acidity of zirconia catalysts containing platinum. European Patent 0174836 describes a similar catalytic method for isomerizing C.sub.6 - normal paraffins.
It has been found that platinum in the presence of hydrogen prevents the solid superacid catalyst from deactivating. While not wishing to be bound by theory, it is believed that Pt hydrogenates the olefinic components produced from carbonium ion intermediates developed in the isomerization reaction. These intermediates are believed to be the precursors for the deactivating residues. T. Hsoi et al., ACS Preprints Symp. Ser. 33(4), pp. 562-567 (1988) describes isomerizing normal C.sub.5 -C.sub.6 paraffins with a so super acid catalyst. Hsoi uses sulfated transition metal oxides, such as sulfated zirconia, which exhibits superacidity and shows catalytic activity even at low reaction temperatures. The acid strengths of the solid superacids were measured by the color change of a Hammett indicator. The Ho acid strengths were found to be greater than or equal to -14.5 and less than or equal to -16.4. Only a small amount of Pt is necessary to improve catalyst activity, catalyst life, and avoid coke deposits. For example, a metal loading of only 0.50 wt. % Pt to a sulfated ZrO.sub.2 catalyst improved catalyst activity remarkably and the catalyst maintained its initial activity after 100 hours of use at conversion temperatures of at least 140.degree. C. The results suggest that a Pt/sulfated ZrO.sub.2 catalyst exhibits the superacidity and high activity necessary to make commercial low temperature catalyzed isomerization reactions possible.
M. W. Wen et al., Energy & Fuels, Vol. 4, pp. 372-379 (1990), studied the formation of highly branched long chain paraffins and highly isomerized short chain paraffins produced from a normal hexadecane feed. The isomerization selectivity observed by Wen was 90 percent in making short chain isoparaffins. The conclusion reached in this study was that Pt/sulfated ZrO.sub.2 catalysts are more effective for hydrocracking heavier normal paraffins, i.e. normal paraffins having four or more carbon atoms, than for hydroisomerizing short chain normal paraffins, based on the relatively low selectivity and the amount of short chain isoparaffins produced (see page 373).
M. Y. Wen et al., A.C.S. Symposium Series, Vol. 35, No. 4, pp. 819-820 (1990), adds methylcyclopentane to a Pt/sulfated ZrO.sub.2 catalyst to improve isomerization activity.
Despite the advances made in solid, superacid isomerization, the catalysts found in the prior art continue to suffer major disadvantages when applied, for example, to long chain hydrocarbons e.g., C.sub.7 +. For instance, large amounts of byproducts, consisting predominantly of cracked hydrocarbon materials, form in prior art isomerization methods along with the desired isomerate product. Cracking reduces the amount of long chain paraffins available for isomerization, thereby reducing the ultimate yield.
In U.S. Pat. No. 4,357,484, Kramer discloses an isomerization process where adamantane is added to halide-containing Lewis Acid catalysts. A detailed look at the isomerization mechanism in Kramer reveals that the catalyst must generate carbonium ion intermediates in solution to effect the isomerization. Therefore, halide-containing Lewis Acid catalysts, such as aluminum bromide, are added to a solvent.
U.S. Pat. No. 3,671,598 describes a commercial process for isomerizing cyclic hydrocarbons with liquid acids such as sulfuric or fluorosulfonic acid in the presence of adamantane.
Under conditions where the isomerization reaction proceeds at rates useful for commercial applications, long chain paraffins (i.e., C.sub.7 + paraffins) crack extensively. Both cracking and isomerization occur at the acid sites of the catalyst. Partially poisoning the catalyst, for example, adding small amounts of aromatics, reduces the amount of undesirable cracked products that form at the expense of reduced productivity.
The term "productivity", used herein, is defined as the weight of paraffinic branched hydrocarbon isomerate produced by the catalyst per weight of catalyst per unit time e.g., g iso C.sub.n /g cat/hr, where n=6 or more, at a particular reaction temperature which ranges from 0.degree. C. to 400.degree. C. Productivity is obtained by multiplying product yield, having the units weight product/weight feed, by the weight hourly space velocity, having the units weight feed/weight catalyst/hour.
An object of the present invention is to provide an economical method for isomerizing long chain paraffins, easily controllable that enables the isomerization to occur productively at low temperatures, and does not involve complex or difficult separation and recovery procedures.
Another object of the invention is to provide an isomerization method where the catalyst can be regenerated readily.
Yet, another object of the invention is to provide a fixed-bed isomerization process that uses a strong acid, solid catalyst with no need for any solvent or liquid phases at reaction conditions, other than the feed. At reaction temperatures the lower boiling feeds will normally be in the vapor phase, but some liquid feed may be present with higher boiling paraffins.